<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        a {
            display: block;
            width: 100px;
            height: 50px;
            line-height: 50px;
            text-decoration: none;
            text-align: center;
            margin: 100px auto;
            border: 1px solid #000;
            color: #000;
            position: relative;
        }
        a:after {
            content: '';
            display: block;
            position: absolute;
            height: 2px;
            left: 50%;
            right: 50%;
            bottom: 5px;
            background-color: skyblue;
            transition: all 0.5s ease;
        }
        a:hover:after {
            left: 0;
            right: 0;

        }
    </style>
</head>
<body>
    <a href="">点击进入</a>
</body>
</html>
